This research is designed to produce a large animal model (lamb) for study of a newborn syndrome characterized by abnormal pulmonary hypertension (commonly known as Persistence of the Fetal Circulation). The model has been produced by chronically embolizing the placenta with microspheres. The pulmonary circulation of lambs born after chronic placental embolization is being studied from the moment of birth. Pulmonary artery pressure and flow, airway pressure, systemic artery and systemic venous pressure are all recorded with appropriate physiologic recorders. Arterial blood gas is determined frequently. Pulmonary vascular responses to changes in inspired oxygen, acid-base status, temperature, hemorrhage, and hypoglycemia can then be measured. This model also permits careful evaluation of the effect of various therapeutic and/or mechanistically informative drugs upon the pulmonary circulation of normal and chronically stressed lambs. Endogenous release of vasoactive substances in response to chronic stress will be measured. Quantitative analysis of the medial musculature of the pulmonary resistance vessels (5th generation arterioles) will be performed in the chronically stressed and compared to normal lamb vessels prepared and studied by a similar technique. BIBLIOGRAPHIC REFERENCES: Fox WW, Gewitz MH, Dinwiddie MB, Drummond WH, and Peckham GJ: Pulmonary hypertension in the perinatal aspiration syndrome. Pediatrics 59:205, February 1977. Drummond WH, Peckham GJ, and Fox WW: Clinical profile of the newborn with persistent pulmonary hypertension. Clin Pediatr, April 1977, in press.